Studies conducted at the University of Missouri Southwest Center in Mt. Vernon, Mo., showed that beef cows allowed access to shade gained 0.72 pounds more/day than those not allowed access to shade, and the pregnancy rate was 87.5 percent for the shaded group compared to about 50 percent for the non-shaded group.
“The best type of shade is the natural type,” according to University of Missouri Extension livestock specialist Andrew McCorkill. “It tends to be darker, cooler and allow for more air flow than most manmade structures.” McCorkill said simple portable shade structures can easily be built at home in the farm shop. “The darker the shade or shading percentage, the better the material for making a shade structure,” he said.  Fifteen to 25 square feet are needed for each calf, and 30-50 square feet for mature cows, to allow for both shade and room for the cows to move.
It’s both the heat and the humidity according to Dr. Glenn Selk, professor of animal science at Oklahoma State University. “The critical temperature at which cattle start to feel more of what we would probably call some kind of heat stress, mild or otherwise, is somewhere in the neighborhood of around 80 degrees,” Selk told Ozarks Farm & Neighbor. At the same time, he said, “As temperature increases a little bit at higher humidities, then the amount of danger to the animals increases correspondingly.”
Selk said research shows heat stress can have a detrimental effect on the breeding ability of both males and females. Even after the air has cooled off, it takes about 60 days for the bull’s sperm cells to go through the maturation process. OSU researchers looked at cows in three environments – 71 degrees with low humidity 24 hours a day, “mild stress” of 97 degrees during the day and 91 degrees at night but at low humidity, and “severe stress” at 98 degrees and 91 degrees and high humidity. They harvested the embryos, and the pregnancy percentages for the respective conditions were 83 percent, 64 percent and 50 percent. “That alone does not tell the whole story,” said Selk, “because as they measured the weight of the embryo and the fluids around the embryo, that weight actually was about half in the stressed animals.”
Endophyte infected tall Fescue can also aggravate the threat from heat stress. McCorkill’s colleague, University of Missouri Extension regional livestock specialist Eldon Cole, told OFN, “Fescue can cause an animal’s body temperature to be elevated to pretty drastic proportions – we certainly know that it will impact rate of gain and their milk production and reproduction; we have a lot of failed breedings as a result of elevated temperatures before that pregnancy is really firmly embedded in that cow’s uterus.”
Cole said cattle producers typically watch for open-mouth breathing, slobbering and high respiration rates as signs of heat stress. “They will do anything they can, certainly in the Fescue case, to cool off,” he said. The animal’s temperature can elevate from the normal 101.5 degrees to 105 degrees and even 108 degrees. “When it gets that hot they don’t feel good, they’re not going to graze; they’re probably more susceptible to any disease risks that might come along,” he said; in addition, those affected by the toxic Fescue often do not shed their winter hair coat.


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